1. Field of the Invention
The present invention relates to a secondary air supply system of an internal combustion engine.
2. Description of Background Art
In a type of internal combustion engines that have been proposed thus far, a valve-lifting system is equipped with a cam shaft and a rocker-arm shaft that pivotally supports a rocker arm disposed in parallel with each other in the cylinder head. In addition, the type of engine has a secondary air valve chamber that is formed in the cylinder head cover to control the supply of secondary air (see, for example, Patent Japanese Patent Application Laid-Open Publication No. 2001-50044).
The engine disclosed in Japanese Patent Application Laid-Open Publication No. 2001-50044 is a four-stroke engine equipped with an SOHC-type valve-lifting system. The cam shaft of the engine is rotatably supported at the interface between cylinder head and the cylinder-head cover, and is positioned on the cylinder axis (on the central axis line of the cylinder bore). In addition, an intake valve and an exhaust valve are disposed respectively at the two sides of the cam shaft.
The intake-side rocker-arm shaft and the exhaust-side rocker-arm shaft are supported in parallel with each other respectively at positions located obliquely above the cam shaft. The two rocker-arm shafts are disposed respectively at the two sides of the cam shaft, so as to be symmetrical with respect to a plane including the cam shaft. Each of the rocker-arm shafts pivotally supports a rocker arm. Each of the rocker arms swings and drives the corresponding one of the intake and the exhaust valves.
The secondary air valve chamber is formed in the cylinder-head cover that covers the cylinder head. The secondary air valve chamber is located on the cylinder axis between the rocker arms respectively located at the two sides of the cylinder axis.
Accordingly, while the two rocker arms are brought into contact with the cam shaft from above, the secondary air valve chamber is disposed further above the two rocker arms. The engine has to have a long dimension in the cylinder-axis direction, resulting in a difficulty in manufacturing a smaller-sized engine of this type.